ref(server): Apply rate limits per item in fast-path

[jan-auer]

Note: This is based on #616 and #617. Only look at the last commit.

Introduces EnvelopeLimiter, a utility that applies rate limits to envelope items. It implements logic around a check function that can either be used to check cached rate limits or Redis. The check function is called with the following rules:

• Once for a single event, if present in the envelope.
• Once for all attachments comprised, unless the event was rate limited.
• Once for all sessions comprised.

Items violating the rate limit are removed from the envelope. This follows a set of rules:

• If the event is removed, all items depending on the event are removed (e.g. attachments).
• Attachments are not removed if they create events (e.g. minidumps).
• Sessions are handled separate to all of the above.

This is only used in the fast-path and will be added to normalization in a follow-up PR.

Changelog (Features): Apply cached rate limits to attachments and sessions in the fast-path when parsing incoming requests. (#618)

[untitaker]

This diff looks fine. I will have to relearn rate limits and quotas sometime soon but the changes here make sense to me